It seems like *maybe* Pleroma is the only #ActivityPub server software that attempts to support C2S, but I can't actually find any current documentation about it. The only attempt at a client I can find is AndStatus, but it's Android only.
The reason I'm looking is that now I have the @posts identity working I would sometimes like to act as that identity (replies and such, mostly) without creating a "blog post". And I don't really feel like cloning the Mastodon server API so I can use an app.